The catalytic gaseous phase oxychlorination of C.sub.2 H.sub.4 with HCl and O.sub.2 (or gases containing same) is quite known and was widely used for the preparation of DCE, from which, in a subsequent stage (by pyrolysis), vinyl chloride is obtained.
Likewise it is known to use the catalytic mass in the form of a fluidized bed, in order to get rid of the considerable heat released by both the oxychlorination reaction and the combustion of ethylene to CO and CO.sub.2 ; the fluidized-bed processes which realized conditions of almost a perfect isothermicity are those which so far have met the greatest success.
Catalytic compositions promoting the oxychlorination have been described. The most common one is based on copper compounds, in general chlorides, supported in different amounts on microspheroidal supports (in general Al.sub.2 O.sub.3) suited, for their granulometric distribution and resistance to friction wear, for use in fluidized beds. The previously described compositions show, however, many drawbacks which limit, and in certain instances hinder, a satisfactory exploitation on an industrial scale. Thus, for instance, there are catalytic formulations characterized by high DCE yields with respect to HCl, which must work, however, with a high C.sub.2 H.sub.4 excess, with respect to HCl, in order to avoid the occurrence of bad fluidization phenomena (at the limit defluidization), wherefore the DCE yields, with respect to ethylene, are rather unsatisfactory. The C.sub.2 H.sub.4 excess, with respect to DCE, cannot, in fact, be converted into DCE and is therefore eliminated with the gaseous exhausts as unreacted C.sub.2 H.sub.4 or in the form of carbon oxides (CO and CO.sub.2).
On the other hand, there are compositions which can work with a lower excess of C.sub.2 H.sub.4, with respect to HCl, without meeting particular problems of bad fluidization of the catalytic bed, thus operating with high DCE yields with respect to C.sub.2 H.sub.4, but which, on the contrary, lead to low HCl conversions. In the latter case, it is necessary to use a special equipment, withstanding HCl, in order to avoid corrosion; moreover, it is necessary to neutralize the unconverted HCl with alkali, with a consequential increase in technical burdens. Moreover, there are compositions characterized by the use of carriers different from Al.sub.2 O.sub.3, for instance SiO.sub.2 or silica-alumina, or by the presence in the catalyst, together with CuCl.sub.2, of other compounds, in general chlorides, of alkali or alkaline-earth metals and of rare earths.
Italian Pat. No. 690,193 teaches the use of compositions based on CuCl.sub.2, alkaline-earth chlorides and rare earth chlorides, in order to reduce the magnitude of the combustion on a fluidized bed. Likewise, British Pat. No. 971,996 exemplified catalytic compositions based on CuCl.sub.2 and alkaline-earth chlorides on either a fixed or fludized bed. None of these compositions has been completely free of all the drawbacks, with respect to a satisfactory industrial exploitation, namely:
an insufficient DCE yield, with respect to C.sub.2 H.sub.4 ; PA1 an insufficient conversion of HCl; PA1 a poor fluidization (at the limit de-fluidization). PA1 a high DCE yield, with respect to C.sub.2 H.sub.4 ; PA1 a high conversion of HCl; PA1 excellent fluidization features.
In order to better appreciate the importance of these drawbacks and, consequently, the importance of their removal, it may be useful to provide more specific information as to the bad fluidization phenomena typical for the catalysts described so far. The reaction may be represented as follows: EQU C.sub.2 H.sub.4 +2HCl+1/2O.sub.2 =Cl--CH.sub.2 CH.sub.2 --Cl+H.sub.2 O,
which means that in order to obtain a 100% DCE yield with respect to C.sub.2 H.sub.4, one must feed, besides oxygen, a mixture of HCl and C.sub.2 H.sub.4 in a molar ratio of at least 2. Thus, for instance, if one feeds HCl and C.sub.2 H.sub.4 to the reactor in a 1.86 molar ratio, the maximum theoretical DCE yield, with respect to C.sub.2 H.sub.4, is: 1.86:2.times.100=93. The yields that can be obtained are usually below this value, because of an incomplete conversion of HCl. In fact, considering: EQU R=HCl/C.sub.2 H.sub.4 molar feed ratio; ##EQU2## it is shown that the DCE yield, with respect to ethylene, is well approximated by the equation: DCE yield=R/2.times.C, so that, in order to obtain high yields, R and C must be both as high as possible. Within a fluidized-bed reactor, consisting of a catalyst based on CuCl.sub.2 supported on microspheroidal Al.sub.2 O.sub.3, prepared according to the known technologies, the following phenomena take place:
with HCl/C.sub.2 H.sub.4 feed ratios relatively low (e.g., lower than 1.9 mols) the fluidization is good, for the reasons hereinabove, the DCE yields, with respect to ethylene, are limited (e.g., below 95%).
increasing the HCl/C.sub.2 H.sub.4 ratio above the previously indicated values, the fluidization worsens and the HCl conversion decreases. Such worsening can be perceived visually in pilot glass reactors and reveals itself by the formation of an increasing number of gas bubbles of growing diameter. When the diameter of the bubbles equals roughly the diameter of the reactor, the "rupture" of the catalytic bed can occur, namely there can be seen inside the bed, the formation of "empty" zones, alternated by "full" zones. Under extreme conditions, the catalyst is dragged out of the reactor.
In industrial reactors the worsening reveals itself in a less striking but still evident way through the enormous loss of catalyst in the cyclones, due to the clogging of the cyclones' legs, which hinders the flowing back, into the catalyst bed, of the catalyst separated at the head of the cyclones. In both cases, it is impossible to carry out normal operations and it is necessary to decrease the HCl/C.sub.2 H.sub.4 feed ratio until a good fluidization is restored or, in extreme cases, to switch off the feed of the reactants.
The causes of these phenomena have been generically ascribed to the so called "stickiness", that hinders the free moving or reciprocal creep of the single granules, within the catalytic bed, because of the formation of clots that are difficult to fluidize (and give rise to the bubbles) and slightly flowable (hence the clogging of the cyclones' legs).
The sticking can be observed only in connection with the increase of the HCl/C.sub.2 H.sub.4 feed ratio and is reversible; all this could mean that what is responsible for said phenomena is the active part of the catalyst, namely Cu (rather than the carrier, whose features are not substantially dependent on said ratio). In fact if we analyze the results of many kinetic works, we trace namely always a mechanism involving:
1. the reaction of C.sub.2 H.sub.4 with Cu.sub.2 Cl.sub.4, in order to give DCE and Cu.sub.2 Cl.sub.2 ; PA0 2. the oxidation of CuCl.sub.2 with O.sub.2 (air), to give Cu.sub.2 OCl.sub.2 ; PA0 3. the reaction of 2HCl with Cu.sub.2 OCl.sub.2 to give again Cu.sub.2 Cl.sub.4 and H.sub.2 O. PA0 (A) Air/C.sub.2 H.sub.4 ratio: such that the O.sub.2 content, in the gaseous exhausts, after condensation of DCE, H.sub.2 O and HCl, be from 3 to 10% by volume. PA0 (B) HCl/C.sub.2 H.sub.4 ratio: the nearest possible to the stoichiometric value (2/1 molar), compatibly with the saving of good fluidization conditions of the catalytic bed and of a sufficiently high conversion of HCl, conditions which depend on the type of catalyst. PA0 (C) Contact time (expressed as a ratio between the volume of the catalytic bed in a fluidized state, and the volumetric flow rate of the mixture of reactants, at the temperature and pressure conditions existing in the catalytic bed): it depends essentially on the type of the catalyst; in general it is from 10 to 40, preferably from 20 to 30 seconds. PA0 (D) Linear velocity of the gases: within the range between the minimum fluidization rate and the dragging speed, both being typical for each type of catalyst; in general said velocity is from 10 to 50, preferably from 20 to 40 cm/s. PA0 (E) Total pressure during the reaction (relevant for achieving an effective contact between the reactants, in a gaseous phase, and the catalyst, in a solid phase); in general pressures greater than atmospheric and up to 600 KPa are used; at greater pressures, energy waste becomes predominant, due to the compression work.
In other words Cu shifts cyclically from Cu.sub.2 Cl.sub.4 (chlorided Cu) to Cu.sub.2 Cl.sub.2 (reduced Cu) to Cu.sub.2 OCl.sub.2 (oxychlorided Cu) and at last again to Cu.sub.2 Cl.sub.4. The equilibrium between such forms depends essentially on the HCl/C.sub.2 H.sub.4 feed ratio. When the ratio is growing, the chlorided form becomes prevailing, while at low values of said ratio the oxychlorided form is prevailing.
The occurrence of the sticking may thus be explained by a predominance of Cu.sub.2 Cl.sub.4 with increasing HCl/C.sub.2 H.sub.4 ratio; on the other hand, there is evidence of the trend of of Cu.sub.2 Cl.sub.4 to form polymers [Kenney C. N., Catal. Rev. Sci. Eng. 11 (2), 197 (1975)]: ##STR1##
One solution of the problems is suggested by Italian Patent Publication No. 25,941 A/81. There is described a method for the preparation of a catalyst free from the drawbacks typical of the catalysts previously described. Said method comprises the impregnation of a preformed catalyst, consisting of CuCl.sub.2 supported on Al.sub.2 O.sub.3, by means of an aqueous solution of MgCl.sub.2, whereupon the catalyst is dried and activated in air at temperatures up to 300.degree. C.; in spite of the excellent performance of this catalyst, it is useful to manufacture catalysts showing even better features.